New trends in anneals feature processing in lamp-based anneal chambers. Such processing calls for accurate temperature assessment at low temperatures. Temperature assessment with mere radiant emission measurements may not be accurate at temperatures below about 400° C. due to a low signal-to-noise ratio. Transmission pyrometry may provide the requisite accuracy and precision.
Transmission pyrometry is a common mode of assessing the thermal state of a substrate (e.g., a silicon substrate). Thermal processing chambers commonly expose a substrate to intense, non-coherent or coherent radiation to raise the temperature of the substrate, either of the whole substrate or a part or surface area of the substrate. The radiation used to heat the substrate creates a strong background radiation environment in the chamber.
High power radiation is used to assess the thermal state of the substrate because it can be differentiated from the background radiation in the chamber. Lasers are typically used because they offer high power, and because they afford the opportunity to select a particular wavelength best suited to the substrate. Lasers produce coherent radiation that, when transmitted through a substrate, can indicate a thermal state of the substrate, which may be registered as a temperature. The transmitted radiation may be detected by a pyrometer, compared to the source radiation, and the result is correlated to infer the substrate thermal state. Heretofore, the source radiation was generally selected to be at a small number (e.g., one or two) of narrow wavelength bands. The transmitted radiation, likewise, was analyzed only at a small number (e.g., one or two) of narrow wavelength bands
There is a need for reliable transmission pyrometric measurements. The detector must be operable in an environment of high radiant noise.